The present invention concerns a type of composite ceramic article having fibres dispersed therein and a method for the manufacturing thereof.
The development of certain techniques using fluids brought to very high temperatures is conditioned to a certain extent by the availability of substances having very low heat conductivity combined with very high resistance to thermal shocks. It is known that these two properties are often incompatible. Ceramic substances have the advantage of having, as a general rule, low heat conductivity; while on the other hand, their resistance to thermal shocks is relatively slight.
It is also known that the thermal shock which a ceramic element having a determined shape can bear is proportional to the breaking strain S and that it is inversely proportional to the product of the coefficient of expansion .alpha. and Young's modulus E. To reduce Young's modulus greatly and thereby to increase the resistance to thermal shocks, it has been proposed to produce ceramic substances retaining a plastic state, but experience shows that the breaking strain S is thereby simultaneously reduced, thus making such a ceramic substance unfit for numerous uses and not improving its resistance to thermal shocks in a very marked way.
It is also known that mats of alumina fibre and zirconia fibre etc . . . , having, for the greater part, a direction of lay perpendicular to the surface of the said mat on which a ceramic substance has been moulded have been produced. Surfaces having very good resistance to thermal shocks combined with excellent mechanical qualities in a predetermined desired direction may thus be provided on request. This method does not enable producing parts having high isotropic mechanical resistance (in several directions) nor the casting of elements having complex shapes having simultaneously good resistance to thermal shocks and an improved mechanical strength in all directions. Now, numerous problems are presented where it would be desirable to be able to protect sensitive elements, intended to be placed in an aggressive medium brought suddenly to a high temperature, against heat and simultaneously against chemical action, while having improved mechanical qualities.
Experiments carried out by the inventors herein have shown that it is possible to produce isotropic ceramic substances having a low Young's modulus and being able to withstand consequently, high thermal shock, by incorporating therein various ceramic fibres dispersed in a cement substance. The improvement in the resistance to thermal shocks becomes particularly apparent when the percentage of ceramic fibres incorporated exceeds 20% of the total weight.